Cross-linking is the process of chemically joining molecules by covalent bonds. This is a common and important process both in nature and in industry, to build large and function-specific molecules from small and simple ones. In the polymer industry, monomers are cross-linked to macromolecular chains which can also be further cross-linked into polymeric networks. In the simplest example, a CH4 molecule can be converted to a CH3 radical by the cleavage of one of its C—H bonds, and two CH3 radicals can then combine themselves to C2H6. Repeating the cleavage of C—H and recombination of carbon radicals can yield a large cross-linked hydrocarbon network, possibly in the form of a thin film. In a conventional cross-linking reaction, precursor molecules containing chemically reactive units to facilitate cross-linking are synthesized and placed together. Another reactive reagent is added to activate the cross-linking reaction; the activation is typically enacted by bond-cleavage and radical formation. Heat or another energy source is typically required to break bonds. To reduce the energy barrier for this bond cleavage and to increase the reaction rate, a catalyst is normally required. In addition, other chemical additives are often used to moderate the reaction rate, and to terminate the reaction after a certain degree of cross-linking is accomplished. Many of these reactive chemical reagents are toxic and environmentally harmful. As such, there is a desire to develop a “green” route of cross-linking so that the use of these chemical reagents can be reduced or eliminated.
To develop such a “green” and practical route of cross-linking, it is relevant to examine the processes for cleaving C—H in an organic precursor molecule. Rupturing and removing a hydrogen atom from a hydrogen-containing molecule is commonly referred as hydrogen abstraction in chemistry. A number of reactants can be used in hydrogen abstraction. Common reactants include hydrogen atom, halogen atom, hydroxyl radical, and other radical species. Although the reactants are reactive, activation energy is still commonly required for hydrogen abstraction and some reactions thus require adequate thermal energy (A. A. Zavitsas, Journal of American Chemical Society 120 (1998)6578-6586). Among these reactants, hydrogen atom is particularly attractive because it is not toxic and its generation is relatively easy. The hydrogen abstraction reaction of using atomic hydrogen to break a C—H bond of an alkane molecule is typically exothermal or energy-neutral but has a transition energy barrier of about 0.5 eV. As such, the reaction rate is relatively low at room temperature. Indeed, for a gas phase reaction of H+CH4→H2+CH3. with a constant supply of both reactants at a partial pressure of 1×10−3 Torr at room temperature, the generation of CH3. to a partial pressure of 10−3 Torr, in the absence of any side reactions, will take about one month. By raising the reaction temperature to 300° C., the same result can be obtained in about 0.3 second. Although similar examples of using thermal energy to drive chemical reactions forward are indeed widely used in industry, this heat-driven approach is not applicable to those reaction systems in which heat causes undesirable side reactions. For polymer manufacturing, heating the polymer above its glass transition temperature will cause undesirable deformation. Novel and economical reaction routes for selective C—H bond cleavage with a high throughput and without any heat requirement are thus desirable.
In another widely adopted method of cross-linking small organic precursor molecules to a polymeric film, the organic precursor molecules are fed into a gaseous plasma powered by a direct-current (DC), radio-frequency (RF) or microwave (MW) energy source. The science of technology of plasma polymerization has been adequately reviewed by pioneers in the field such as Yasuda (H. Yasuda, Plasma Polymerization, Academic Press, Inc., New York, 1985), Biederman (H. Biederman, edited, Plasma Polymer Films, Imperial College Press, London, 2004), and Fridman (A. Fridman, Plasma Chemistry, Cambridge University Press, New York, 2008). It is commonly recognized that even when pure organic precursor molecules are fed into plasma, the plasma chemistry is complex and many different bond-breaking processes are active in the plasma. In essence, when plasma is ignited in a gas, some atoms and molecules in the gas are ionized to generate a large number of electrons and ions. Typically these electrons can have an average energy of a few electron volts and a broad energy distribution. Expressed in an equivalent value in temperature, these electrons can reach 105 K. In the plasma, they diffuse much more quickly than ions and their frequent collisions with the atoms and molecules in the plasma lead to excitation, ionization, and bond dissociation. The relaxation of some of these excited species can emit light including ultraviolet light which can also cause secondary excitation, ionization, and bond dissociation. Hence, although a polymer film can be practically formed with plasma polymerization, it is difficult to control the resultant film to match a specific chemical specification such as a film having only one type of chemical functional group (e.g., COOH) in a certain desirable concentration (e.g., one COOH group per three carbon atoms such as that in polyacrylic acid). In fact, Yasuda wrote, “most organic compounds with oxygen-containing groups such as —COOH, —CO—, —OCO—, —OH, and —O—, are generally reluctant to form a polymer, and the plasma polymers rarely contain the original oxygen-containing groups” (H. Yasuda, Plasma Polymerization, Academic Press, Inc., New York, 1985; pp. 112-113).
Several special plasma polymerization methods have been developed to address these limitations of the general plasma polymerization methods. For example, the technique of pulsed plasma polymerization has been developed to harness the complex processes of excitation, ionization, and dissociation in the plasma by supplying the plasma energy to the reactant gas in a train of pulses with controls of the duration, frequency and power of the pulses. The concept and applications of this technique have been explained by Friedrich et al. (J. Friedrich, W. Unger, A. Lippitz, I. Koprinarov, A. Ghode, S. Geng, G. Kuhn, “Plasma-based introduction of monosort functional groups of different type and density onto polymer surfaces. Part 1: Behaviour of polymers exposed to oxygen plasma”, Composite Interface 10(139-171) 2003; and “Part 2: Pulsed plasma polymerization”, ibid 10(173-223)2003). In their work, monomer precursor molecules having a C═C bond such as acrylic acid (H2C═CHCOOH) receive a short pulse of plasma energy and undergo excitation, ionization and dissociation. Although undesirable reactions leading to the loss of the —COOH functional groups will inevitably occur, most of these undesirable reactions cease during the pulse-off-cycle. However, the polymerization chain reaction in cross-linking acrylic acid molecules persists even when the plasma pulse is off. In an optimized pulsed plasma polymerization process, when the cross-linking chain reaction runs out of stream, the plasma pulse is applied to prime the chain reaction again. For example, Friedrich et al. have demonstrated that up to 73% of the —COOH in acrylic acid can be retained in a polymer film formed by this pulsed plasma polymerization method. Since the loss of useful functional group and the formation of undesirable functional groups can still occur when the plasma pulse is on, an alternative technique to eliminate these problems is still desirable.
In the research and development of new reaction routes, scientists have discovered that the kinetic energy of a reactant can be an important reaction attribute. It can be used to drive a chemical reaction which otherwise relies totally on the thermal energy supplied to the reaction system and the chemical potentials of reactive chemical reagents. The best fundamental evidence can be found in most scientific articles on molecular beam research in the literature (see for example, M. A. D. Fluendy and K. P. Lawley, “Chemical applications of molecular beam scattering”, Chapman and Hall, 1973). In this research, a beam of atoms or molecules having a specific kinetic energy and internal energy is directed to a target. The energy exchange and resultant chemical reactions are examined. Such experiments are, however, technically demanding and economically expensive. In a typical molecular beam experiment, kinetic energy is added to the atoms or molecules when they are adiabatically expanded with an inert gas through a small nozzle. The velocity of the atoms or molecules can increase to supersonic speed. However, this technique is not suitable for light species, since the kinetic energy of a light molecule like hydrogen traveling at supersonic speed is still much less than 0.1 eV. Although it is possible to speed up a heavy hydrogen-containing molecule such as HI and split it with a laser beam for the formation of hyperthermal atomic hydrogen, this is certainly not a practical method to practice C—H bond cleavage in industry.
The kinetic energies of the atoms or molecules can also be increased by ionizing them and then accelerating them using an electrostatic ion acceleration process. These accelerated ions can be used to bombard a target in an “ion bombardment” process. Many industrial processes indeed use ion bombardment to reduce the reliance of synthetic reactions on thermal energy and to promote reactions via non-thermal equilibrium pathways (see for example, O. Auciello and R. Kelly, “Ion bombardment modification of surfaces”, Elsevier Science, 1984). In practice, ion bombardment of an electrically insulating surface is not practical because of surface charging. Although many analytical instruments such as ion microscopes circumvent such surface charging problems by flooding the ion bombarded area with low energy electrons, the concurrent supplies of both energetic ions and electrons with precise controls in energy and dosage to a large irradiation area for practical industrial manufacturing are technically challenging and economically expensive.
Recently Lau and coworkers have shown that bombarding an organic molecule with hyperthermal proton can preferentially break C—H bonds without breaking other bonds (R. W. M. Kwok and W. M. Lau, “Method for selectively removing hydrogen from molecules”, US Patent Application 20030165635, filed Feb. 25, 2003; L. Xi, Z. Zheng, N. S. Lam, H. Y. Nie, O. Grizzi, and W. M. Lau, “Study of the hyperthermal proton bombardment effects on self-assembled monolayers of dodecanethiol on Au(111)”, J. Phys. Chem. C 112, 12111-12115 (2008); C. Y. Choi C Y, Z. Zheng, K. W. Wong, Z. L. Du, W. M. Lau, and R. X. Du R X, “Fabrication of cross-linked multi-walled carbon nanotube coatings with improved adhesion and intrinsic strength by a two-step synthesis: electrochemical deposition and hyperthermal proton bombardment”, Appl. Phys. A 91, 403-406 (2008); W. M. Lau, Z. Zheng, Y. H. Wang, Y. Luo, L. Xi, K. W. Wong, and K. Y. Wong, “Cross-linking organic semiconducting molecules by preferential C—H cleavage via “chemistry with a tiny hammer”, Can. J. Chem. 85, 859-865 (2007); L. Xi, Z. Zheng, N. S. Lam, O. Grizzi, and W. M. Lau, “Effects of hyperthermal proton bombardment on alkanethiol self-assembled monolayer on Au(111)”, Appl. Surf. Sci. 254, 113-115 (2007); Z. Zheng K. W. Wong, W. C. Lau, R. W. M. Kwok and W. M. Lau, “Unusual kinematics-driven chemistry: cleaving C—H but not COO—H bonds with hyperthermal protons to synthesize tailor-made molecular films”, Chem. Euro. J. 13, 3187-3192 (2007); Z. Zheng, W. M. Kwok, and W. M. Lau, “A new cross-linking route via the unusual collision kinematics of hyperthermal proton in unsaturated hydrocarbon: the case of poly(trans-isoprene)”, Chem. Comm. 29, 3122-3124 (2006); X. D. Xu, R. W. M. Kwok, and W. M. Lau, “Surface modification of polystyrene by low energy hydrogen ion beam”, Thin Solid Films 514, 182-187 (2006); Z. Zheng, X. D. Xu, X. L. Fan, W. M. Lau, and R. W. M. Kwok, “Ultrathin polymer film formation by collision-induced cross-linking of adsorbed organic molecules with hyperthermal protons”, J. Amer. Chem. Soc. 126, 12336-12342 (2004)).
The novelty of this approach is the exploitation of the unusual kinematics when a hyperthermal proton strikes an organic molecule adsorbed on a conductive solid substrate. In this bombardment process, the incoming proton will first be neutralized by the conductive substrate when it is still >0.5 nm above the surface. The neutral atomic hydrogen projectile carrying a few eV in kinetic energy continues to approach the target organic molecule and enters first to the attractive chemical potential region and forms a transient molecule with the target. The kinetic energy then drives the projectile into the repulsive potential region and finally the projectile uses up its kinetic energy. If the projectile and target are merely two hard spheres, after the closest encounter they will fly apart and the maximum energy transfer is determined by the two masses with the formula: 4MpMt(Mp+Mt)2. Hence, a projectile of an atomic mass unit of one can transfer its kinetic energy very effectively to a target of an atomic mass unit of one (hydrogen atom) but the maximum kinematic energy transfer drastically drops to 28% if the target has an atomic mass unit of twelve (carbon atom). This difference in kinematic energy transfer can be exploited, in principle, to preferentially break C—H bonds because the typical dissociation energy of C—H and other sigma bonds of an organic molecule is 4-5 eV. Indeed, Lau and co-workers have demonstrated the feasibility of this concept by using protons of less than 20 eV to break C—H bonds without breaking other bonds in a variety of organic molecules.
By condensing polyacrylic acid as the precursor molecules on a silicon wafer surface, they have demonstrated the cross-linking of them into a stable molecular layer with retention of more than 95% of the —COOH group by their proton bombardment method. In all their published experimental data, protons are used because protons can be attracted from hydrogen plasma and the proton energy can be controlled quite precisely with the common techniques of ion optics. They have also confirmed the theoretical validity of the concept by ab initio molecular dynamics computations for the collisions of a proton with a simple hydrocarbon molecule under different collision trajectory conditions. Their published results are informative in laying the foundation of using kinematic energy transfer to break C—H bonds, but the approach of proton bombardment suffers the same surface charging problems of all ion bombardment techniques and is not practical for the industrial manufacturing of polymeric products.
Therefore, it would be very advantageous to provide a method of selectively breaking C—H and/or Si—H bonds which can be used with any substrate and which avoids breaking other molecular bonds.